1. Field of the Invention
This invention relates to the art of filtering particulate solids and other matter from liquids, and more specifically, to an improved method and apparatus for filtering with a granular medium filter such as a sand filter.
2. Information Disclosure Statement
Granular medium filters such as sand filters are widely used to remove particulate material and other matter from wastewaters, potable water supplies, and the like. Such filters trap fine particulate matter in the interstices between granules in the upper portion of the filter bed, and larger size particles form a layer on the filter surface. Eventually, the flow of liquid through the filter bed is hindered so that the liquid level above the bed rises. The plugging materials are removed periodically by stopping the flow of influent water and backwashing with already-filtered water. Usually, the filter is backwashed when the liquid level or liquid head above the filter bed reaches a predetermined height.
Influent waters typically vary in suspended solids content, size and nature, as a function of time. In most cases, the flow rate or hydraulic load also widely varies with time. Such variations result in frequent backwashing during some periods, and less frequent backwashing at other times. The frequency variation is sometimes diurnal, but often cannot be predicted with much certainty. Higher concentrations of fine particles in the influent water lead to faster clogging and the need for more frequent backwashing.
Extending the filtration time between backwashing has several advantages. First, downtime is minimized, thus maximizing effective filtration time. Second, the total quantity of solids loaded onto the filter bed between backwashes is increased. With a constant volume of backwash water, the solids concentration in the backwash water is increased. This reduces the overall hydraulic load placed on subsequent backwash water treatment equipment. Whenever backwash water is recycled to the head end of a treatment plant, it adds to the hydraulic load and solids load of the plant. The backwash water is more easily treated when it comprises a small volume containing concentrated solids. This is best accomplished by longer filter runs between backwashes.
Several methods have been used to prolong the filter run length between backwashes, without using additional filter area. Ross U.S. Pat. No. 3,459,302 discloses a granular medium filter in which currents are created in the liquid above the filter bed surface by an air diffuser. These currents sweep across the surface, removing solids and maintaining them in suspension in the liquid above the filter surface.
Another very effective means for reducing the frequency of backwashing is described in Ross U.S. Pat. No. 3,817,378. In this reference, at the time a filter bed begins to clog, streams of air are forced upward through the medium in intermittent pulses of short duration. Some variations of this procedure are known as "air scour". A portion of the filtered solids is forced into liquid suspension, while another portion is concentrated by sub-fluidizing medium movement into localized sites within the bed itself. Thus, some of the solids are "stored" within the filter bed, reducing the quantity of solids which produce the flow resistance at the bed surface. This filter cleaning operation is generally repeated a number of times between backwashes, greatly extending the filtration time before backwashing is required.
The quantity of filtered solids which may be stored in the bed without adversely affecting the filtration rate is limited. This generally limits the number of pulses which may be advantageously performed between backwashes.
As currently practiced, the air scour or pulse system is activated when the liquid level above the filter bed rises to activate a first level detector. This detector is generally situated at a low position in the filter tank. Additional pulses are initiated on a timed basis at present intervals of 6-30 minutes, depending upon the expected solids loading and hydraulic loading. Backwashing is typically initiated when a second level detector, located at a level higher in the filter tank, is activated by the high liquid level.
Influent liquids with a high solids content tend to clog the filter rapidly, requiring more frequent pulsing as well as more frequent backwashing. A given present pulse interval is optimal only at a single solids loading rate. In most situations, the solids loading rate varies rapidly and widely with time, so that a given constant pulse interval alternates between being too short and being too long. This inefficient operation increases the frequency of backwashes and/or greatly increases the number of unneeded pulses between backwashes.